[flang] Fix for broken/degenerate forall case

[clementval]

Summary

Fix for broken/degenerate forall case where there is no assignment to an
array under the explicit iteration space. While this is a multiple
assignment, semantics only raises a warning.
The fix is to add a test that the explicit space has any sort of array
to be updated, and if not then the do_loop nest will not require a
terminator to forward array values to the next iteration.

This patch is part of the upstreaming effort from fir-dev branch.

Co-authored-by: Eric Schweitz <[e-mail]>

flang/lib/Lower/Bridge.cpp

auto lambda = [&, lows, highs, steps]() {
                    ? builder->createConvert(
                          loc, idxTy,
                          lowerExpr(*Fortran::semantics::GetExpr(*optStep)))
                    : builder->createIntegerConstant(loc, idxTy, 1);
         }
         auto lp = builder->create<fir::DoLoopOp>(
             loc, lb, ub, by, /*unordered=*/true,
             /*finalCount=*/false, explicitIterSpace.getInnerArgs());
-        if (!loops.empty() || !outermost)
+        if ((!loops.empty() || !outermost) && !lp.getRegionIterArgs().empty())
           builder->create<fir::ResultOp>(loc, lp.getResults());
         explicitIterSpace.setInnerArgs(lp.getRegionIterArgs());
         builder->setInsertionPointToStart(lp.getBody());
         forceControlVariableBinding(ctrlVar, lp.getInductionVar());
         loops.push_back(lp);
       }
       if (outermost)
         explicitIterSpace.setOuterLoop(loops[0]);
std::visit(
               auto lhs = [&]() -> fir::ExtendedValue {
                 if (lhsIsWholeAllocatable) {
                   lhsMutableBox = genExprMutableBox(loc, assign.lhs);
                   llvm::SmallVector<mlir::Value> lengthParams;
                   if (const fir::CharBoxValue *charBox = rhs.getCharBox())
                     lengthParams.push_back(charBox->getLen());
                   else if (fir::isDerivedWithLenParameters(rhs))
                     TODO(loc, "assignment to derived type allocatable with "
-                              "length parameters");
+                              "LEN parameters");
                   lhsRealloc = fir::factory::genReallocIfNeeded(
                       *builder, loc, *lhsMutableBox,
                       /*shape=*/llvm::None, lengthParams);
                   return lhsRealloc->newValue;
                 }
                 return genExprAddr(assign.lhs, stmtCtx);
               }();
 

flang/lib/Lower/ConvertExpr.cpp

void lowerAllocatableArrayAssignment(const Fortran::lower::SomeExpr &lhs,
     }();
 
     llvm::SmallVector<mlir::Value> lengthParams;
     // Currently no safe way to gather length from rhs (at least for
     // character, it cannot be taken from array_loads since it may be
     // changed by concatenations).
     if ((mutableBox.isCharacter() && !mutableBox.hasNonDeferredLenParams()) ||
         mutableBox.isDerivedWithLenParameters())
-      TODO(loc, "gather rhs length parameters in assignment to allocatable");
+      TODO(loc, "gather rhs LEN parameters in assignment to allocatable");
 
     // The allocatable must take lower bounds from the expr if it is
     // reallocated and the right hand side is not a scalar.
     const bool takeLboundsIfRealloc = rhs.Rank() > 0;
     llvm::SmallVector<mlir::Value> lbounds;
     // When the reallocated LHS takes its lower bounds from the RHS,
     // they will be non default only if the RHS is a whole array
     // variable. Otherwise, lbounds is left empty and default lower bounds